Add Package Search

I know this was addressed in issue #101, but I feel it is very suboptimal.

I'm an (admittedly new) user and found myself searching for an older version of `errors` (required by another package). There was no way for me to search for an older version of that package on stackage. The `browse packages` page also does not provide an intuitive way to switch between LTS versions to find the older version by searching each page.

It would be great to allow for package searches that include specifying the package or LTS version. It also would be nice to have https://www.stackage.org/lts-3.3 have some sort of option for choosing different LTS versions (similar to the upper left corner of the [Python Docs](https://docs.python.org/3.4/library/string.html) page).